1. Field of the Invention
The present invention relates to a toner quantity measuring apparatus which measures the quantity of toner adhering to an image carrier such as a photosensitive member and a transfer medium, and an image forming apparatus which comprises such a toner quantity measuring apparatus.
2. Description of the Related Art
For the purpose of realizing a stable image density, an image forming apparatus of the electrophotographic type, such as a printer, a copier machine and a facsimile machine, internally comprises a toner quantity measuring apparatus which measures the quantity of toner adhering to an image carrier such as a photosensitive member and a transfer medium. Such a toner quantity measuring apparatus is as described in Japanese Patent Application Unexamined Gazette No. 2000-29271, for example. A toner quantity measuring apparatus described in this gazette (hereinafter xe2x80x9cfirst conventional apparatusxe2x80x9d) has a light emitting element irradiating light toward an image carrier such as a photosensitive member and a reflection-side light receiving unit including a light receiving element. The light receiving element receives reflected light from the photosensitive member so that the quantity of toner on the photosensitive member is calculated based on the quantity of the received light (the quantity of the reflected light).
Further, aiming at stabilization of the quantity of the irradiated light, a beam splitter splits the irradiated light at a predetermined ratio, whereby the irradiated light is partially extracted. Another light receiving element (of irradiation-side light receiving unit) detects the quantity of the extracted light, and the light emitting element is feedback-controlled in such a manner that the detection result stays at a reference value.
Meanwhile, often used as a light receiving element is as shown in FIG. 1, for instance. FIG. 1 is a drawing of an electric structure of a conventional light receiving unit. In this light receiving unit, an anode terminal of a light receiving element PS, such as a photodiode, is connected with a ground potential and a non-inversion input terminal of an operational amplifier OP which forms a current-voltage (I/V) conversion circuit. A cathode terminal of the light receiving element PS is connected with the non-inversion input terminal of the operational amplifier OP, and additionally, with an output terminal of the operational amplifier OP through a resistor R. Hence, as the light receiving element PS receives light and carries a photoelectric current i, an output voltage V0 at the output terminal of the operational amplifier OP is:
V0=ixc2x7R
Thus, the light receiving unit outputs a signal corresponding to the quantity of the reflected light.
In the light receiving unit having such a structure, since the level of the output signal, e.g., an output voltage, from the light receiving unit changes approximately in proportion to the quantity of incident light which is the quantity of the reflected light from a photosensitive member, the circuitry of the light receiving unit is normally configured such that a detection signal having a characteristic as that denoted at the solid line in FIG. 2 is obtained. However, depending on irregularity among light receiving units or other circuit elements, a change in characteristics due to an environmental condition, a change in characteristics due to deteriorated durability, etc., a characteristic as that denoted at the dotted line or the dotted-and-dashed line in FIG. 2 may be realized.
Now, a characteristic as that denoted at the dotted-and-dashed line in FIG. 2 will be considered. Assuming that the circuit shown in FIG. 1 is operated by a dual power supply which uses a (+15V)-power source and a (xe2x88x9215V)-power source, a negative voltage is outputted when the quantity of the reflected light is zero. However, as a dual power supply requires a higher cost for a power source part, a single power supply with only a (+15V)-power source is often used in an actual apparatus. Yet, if only one power source is used, as indicated by the characteristic at the dotted-and-dashed line in FIG. 2, a so-called dead zone where the output voltage level remains at zero without any change will be developed. This in other words is a problem that such a toner quantity which produces only a small amount of reflected light can not be measured. This problem worsens particularly when high-density black toner is to be detected, since black toner absorbs light and the amount of reflected light therefore sharply decreases.
Noting this, another option for measurement of a toner quantity on the high-density side may be to increase the quantity of the irradiated light from the light emitting element, and hence, the quantity of the reflected light. However, this merely shifts the problematic zone but fails to completely solve the problem since a similar problem will rise during measurement of the quantity of toner having an even higher density. Further, in the case of the first conventional apparatus, it is possible to set the quantity of the irradiated light from the light emitting element only at one single light quantity. Hence, a toner quantity can be accurately measured only within a limited density range in the first conventional apparatus.
On the other hand, a characteristic as that denoted at the dotted line in FIG. 2 leads to a situation that an output does not become zero even if the light emitting element is not irradiating light, which is known as outputting of a dark output. Due to this, even when the light emitting element irradiates light upon the photosensitive member and the quantity of the reflected light from the photosensitive member is detected, the detection result contains a dark output component. Adding to the difficulty, the dark output is relevant to characteristics such as a dark current of the light receiving unit and an offset of the operational amplifier, and therefore, changes in accordance with an environmental condition, such as a temperature around the apparatus, and a change with time of the components which form the apparatus. Thus, highly accurate measurement of a toner quantity is difficult.
A conventional approach to these problems is to suppress the irregularity using an adjustment circuit which is disposed inside the apparatus. However, such a structure has been met with a challenge that the light receiving unit has a complex circuit, a higher cost is required as repeated adjustment is necessary and even more highly accurate measurement is difficult because of other factors such as uneven adjustment.
In a different toner quantity measuring apparatus described in the gazette above (hereinafter xe2x80x9csecond conventional apparatusxe2x80x9d), a light emitting element irradiates light toward a photosensitive member (image carrier), light reflected at the photosensitive member is split into p-polarized light and s-polarized light, and a p-polarized light receiving unit detects the quantity of the p-polarized light while an s-polarized light receiving unit detects the quantity of the s-polarized light. The quantity of toner on the photosensitive member is found based on a difference between these two light quantities.
In the second conventional apparatus, units as that shown in FIG. 1 are used as the light receiving units, which results in similar problems to those with the first conventional apparatus described above. Further, measuring the quantity of the toner based on the difference between the two light quantities, the second conventional apparatus has another problem as described below. Owing to an environmental factor such as an ambient temperature and humidity, a change with time of the light emitting element, etc., the quantity of irradiated light upon the photosensitive member, a transfer image carrier or the like may sometimes change, and therefore, a toner quantity is wrongly detected because of the change in the quantity of irradiated light. For instance, as the quantity of irradiated light upon an image carrier such as the photosensitive member decreases, the quantities of the p-polarized light and the s-polarized light as well decrease, thereby changing the light quantity difference. As a result, a toner quantity calculated based on the difference as well changes, which worsens a measurement accuracy.
In addition, while color toner and black toner adhere to an image carrier such as a photosensitive member and a transfer medium in a color image forming apparatus, color toner and black toner have different reflection characteristics from each other. Thus, for measurement of the quantity of toner based on the quantity of reflected light, a toner quantity should be measured optimally for each toner color. Despite this, merely one type of toner quantity measurement is executed according to the first and the second conventional techniques, leaving enough room for improvement in measurement accuracy.
A principal object of the present invention is to provide a toner quantity measuring apparatus which allows highly accurate measurement of the quantity of toner which adheres on an image carrier such as a photosensitive member and a transfer medium.
Another object of the present invention is to provide an image forming apparatus which creates an image with a stable density based on a result of measurement obtained by the toner quantity measuring apparatus.
In fulfillment of the foregoing object, a predetermined output offset is applied to the output from a light receiving element. Toner quantity calculating means calculates the quantity of toner which adheres to an image carrier based on the output from the light receiving element. In this manner, with application of the output offset, it is possible to eliminate an influence of a dead zone without fail and output an output which corresponds to the quantity of the reflected light.
According to another aspect of the present invention, irradiation amount adjusting means keeps a light emitting element turned off while a light quantity control signal to control the quantity of light irradiated by the light emitting element remains below a predetermined input offset. This allows to turn the light emitting element off without fail.
According to further aspect of the present invention, reflection quantity detecting means detects light quantities of a first and a second light components which are different from each other and contained in reflected light from an image carrier, and toner quantity calculating means calculates a light quantity ratio between the light quantity of the first light component and the light quantity of the second light component which are detected by the reflection quantity detecting means, and calculates the quantity of toner adhering on the image carrier based on the light quantity ratio.
According to still another aspect of the present invention, toner quantity calculating means is structured so as to be able to execute, as a measurement process of measuring a toner quantity, a plurality of toner quantity detection processes which are different from each other, and selectively executes one of the plurality of toner quantity detection processes in accordance with the color of toner adhering on the image carrier.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawing. It is to be expressly understood, however, that the drawing is for purpose of illustration only and is not intended as a definition of the limits of the invention.